Alerted by the Jimmy Ruffin fan, I have to point out two more who don't belong in this category either.

Freda Payne had several other big records on the radio, and 'Bring The Boys Home' was a bona fide hit, an even better song than 'Band of Gold'

For Joe South, 'Games People Play' may well be the song that earns him the most in performance income from radio play, but 'Walk a Mile in My Shoes' was a very big hit too, and I think he had the first hit version of his own song 'Hush', which reappeared several times since, by Billy Joe Royal, Deep Purple and Kula Shaker. One of the great singer-songwriters of his generation (he also wrote 'I Never Promised You a Rose Garden'), Joe definitely shouldn't be classed in this category.

Maybe you need another category - people who look like one hit wonders at first glance but turn out to be substantial writers/producers/etc

i've been using iTunes to buy one hit wonders of my childhood. Here's a few:

Berlin - Take My Breath Away
Ace Frehley - New York Groove
Arlo Guthrie - City Of New Orleans
Climax Blues Band - Couldn't Get It Right
Stargard - Which Way Is Up (disco was great for one hit wonders!)